1. Field of the Invention
The present invention relates to an apparatus for developing an electrostatic image, and more particularly to a developing apparatus wherein a magnetic field is formed for using a magnetic developer (hereinafter also called magnetic toner) for the image development and the thickness of layer of said toner on a toner carrying member can be limited.
2. Description of the Prior Art
The conventional methods employed in the developing apparatus in electrophotography and electrostatic recording can be classified into dry developing methods and wet developing methods. The former can further be classified into those utilizing a two-component developer and those utilizing a single-component developer. The two-component methods include, according to the classification by the species of carrier employed in combination with the toner, a magnetic brush method utilizing iron power carrier, a cascade developing method utilizing bead carrier, a fur brush method utilizing a fur brush etc. Also the single-component methods include a powder cloud method in which the toner particles are used in a state of sprayed cloud, a contact development or toner development method in which toner particles are brought into direct contact with a surface carrying electrostatic image, a jumping development method in which the toner particles are not brought into direct contact with said surface but are electrically charged and made to fly toward said surface by the electric field of the electrostatic image, a magnetic dry developing method in which magnetic electroconductive toner particles are brought into contact with the surface carrying the electrostatic image etc.
The two-component developing methods employing a mixed developer consisting of carrier particles and toner particles of which the latter are consumed in much larger proportion than the former to alter the mixing ratio of said particles with the progress of development, is fundamentally associated with the drawbacks of the fluctuation of image density resulting from said change in the mixing ratio and the deterioration of image quality resulting from the deterioration of carrier particles which remain unconsumed for a prolonged period.
On the other hand, among the single-component developing methods, the magnetic dry developing method utilizing magnetic toner and the contact developing method not utilizing magnetic toner both involve indiscriminate contact of toner particles with the entire surface to be developed, image area and non-image area inclusive, which tends to cause toner deposition even in the non-image area, thus resulting in so-called background fog. Such background fog is also unavoidable in the two-component developing methods. Also the powder cloud method is associated with said background fog resulting from deposition of toner particles in powder cloud state onto the non-image area.
As a single-component developing method there is already known so-called jumping development method as disclosed in the U.S. Pat. Nos. 2,839,400 and 3,232,190 wherein a toner carrying member such as a sheet member uniformly coated with toner particles is maintained at a small distance from a surface carrying an electrostatic image, and the toner particles are attracted from said carrying member toward said image carrying surface by means of the electric charge of electrostatic image thereby performing the development of said image. The above-mentioned method is advantageous in that it is almost free from the above-mentioned background fog as the toner particles is not attracted in the non-image area having no electrostatic charge nor brought into contact with the non-image area, and also in that it is free from the above-mentioned change of mixing ratio and also from the deterioration of carrier particles since there are no carrier particles involved.
However, the above-mentioned method have been associated with other drawbacks which are enumerated in the following:
(1) Difficulty in obtaining practically uniform toner coating. Uniform toner deposition is difficult to obtain although the toner carrying sheet is previously provided with an electric field to facilitate toner deposition. Different from liquid coating, a thin and uniform coating of particles is difficult to achieve for example with a known rigid blade. Uneven coating is not suitable for practical image reproduction as the unevenness is directly reproduced on the developed image. As an improvement it has been proposed to use a cloth or paper as the toner carrying sheet and embedding the toner particles into the fibers thereof, but it is still difficult to obtain a uniform coating as the toner particles finer than the fiber size is difficult to prepare. Also the toner deposition onto the toner carrying sheet by cascade developing method is not practical as it requires a large-sized apparatus.
(2) Difficulty in uniform toner removal from the toner carrying member. The coated toner layer, when brought into facing relationship with the electrostatic image, should cause uniform removal and transfer of toner as otherwise uniform image development cannot be expected. Such uniform removal of toner particles depends on the surface characteristics of sheet carrying the toner particles, the coating condition thereon and the characteristics of toner particles, and has never reached the practically acceptable level.
(3) Low image resolution.
In the known jumping development method the toner particles are electrostatically deposited on the toner carrying member, and, even if a relatively thin toner layer is formed on said carrying member, the toner particles are considered to fly toward the surface carrying the electrostatic image by the mutually repulsive charges of said toner particles when the distance to said surface is reduced to approximately 3 mm. However the flight of toner particles over such a wide distance from the toner carrying surface to the image holding surface requires a long time and tends to be influenced by the air stream flowing through the gap, the toner gravity, and the eventual vibration of image carrying surface or toner carrying member, giving rise to deterioration of developed image. Also the electric field of fine lines or fine characters in the electrostatic image does not exactly reach the toner carrying surface, so that there may result thinning of fine lines or fine characters, or significant deterioration of resolution due to the lack of flight of toner particles. On the other hand if said distance is rendered too small, it is again difficult to obtain exact reproduction as the fine lines or fine characters tend to become thicker.